1. Field of the Invention
The present invention relates to an image forming apparatus such as a facsimile, a printer, a copier, and a multifunction peripheral.
2. Description of the Related Art
Image forming apparatuses such as a facsimile, a printer, a copier, and a multifunction peripheral perform a sequence of image forming operations in the following manner. A photosensitive element (which is uniformly charged in advance by a charging unit) as an image carrier that rotates in a sub scanning direction is exposed by an exposing unit in response to an original image or image data to form an electrostatic latent image. Toner is adhered to the latent image by a developing unit to form a toner image. The toner image is transferred by a transfer unit onto a sheet as a recording medium. The toner image on the sheet is then fixed thereto by heat and pressure by a fixing unit including a fixing roller with a built-in heater and a pressing roller.
It is known that, in such an image forming apparatus, the condition of the surface of the photosensitive element or image formation is changed due to an increase in temperature of the photosensitive element surface, which leads to an image defect.
The increase in temperature of the photosensitive element surface is caused by friction between the photosensitive element and a developing roller or the transfer unit near the photosensitive element, frictional heat produced by rotation of the relevant rollers, or heat from the fixing unit.
The image defect caused by the increase in temperature of the photosensitive element surface is due to a scratch on the photosensitive element surface when the photosensitive element surface has a temperature higher than a predetermined temperature.
When the photosensitive element surface is scratched, the development cannot be performed thereafter on the scratched portion. Accordingly, printing onto a recording sheet (also referred to as “paper”, “recording medium”, or “printing material”) produces faulty images with image quality of a part of an image at a position corresponding to the scratched portion degraded.
The degrees of the increase in temperature of the photosensitive element surface are different in a case in which no recording sheet is passed through like in a warm-up operation, and in cases of single-side printing and duplex printing. This is because air flows in different directions in these cases due to the configuration inside the image forming apparatus (interior configuration). Accordingly, timings of occurrence of the image defect are not uniform.
The speed at which the temperature of the photosensitive element surface increases becomes higher in order of the warm-up operation, the single-side printing, and the duplex printing. The speed at which the temperature of the heated photosensitive element surface cools down becomes higher in the reverse order.
This is because the recording sheet that is being subjected to printing blocks an outlet of air in the apparatus, so that heat stays in the apparatus.
In the case of duplex printing, because images are printed on both sides of a recording sheet through two conveying paths in the apparatus, heat is particularly prone to stay in the apparatus, and thus the temperature of the photosensitive element surface increases more quickly.
To overcome the image defect, a technique is applied that enables to measure the temperature of the photosensitive element surface, and execute or stop printing according to the temperature condition, thereby preventing the problem from occurring.
FIG. 10 is a graph representing a change in temperature of the surface of the photosensitive element in the image forming apparatus during printing. In this example, duplex printing is continuously performed. In FIG. 10, an image error occurs (with high possibility) when the surface has a temperature equal to or higher than a temperature indicated by a dashed line (for example, 44° C.).
When continuous printing is performed and the photosensitive element is continuously operated for a time period α (about 55 minutes) in FIG. 10 at 28° C., which is a room temperature within a range in which the image forming apparatus can form appropriate images (corresponding to the temperature outside of the image forming apparatus), the surface temperature reaches the temperature (44° C.) at which the image defect occurs. In this case, the printing is performed when the temperature of the photosensitive element surface is lower than 44° C., and the printing is stopped when the temperature is equal to or higher than 44° C.
Some conventional image forming apparatuses measure the temperature of the photosensitive element surface or the fixing temperature, and enable printing based on the measurement result (for example, see Japanese Patent Application Laid-open No. H05-241405, Japanese Patent Application Laid-open No. H01-172853, and Japanese Patent Application Laid-open No. S58-116544).
According to these image forming apparatuses, the condition of the temperature of the photosensitive element surface can be reflected on the printing status as mentioned above, and thus the image defect can be easily overcome.
However, these conventional image forming apparatuses need a sensor for measuring the temperature (hereinafter, “temperature sensor”). Therefore, manufacturing costs of the image forming apparatuses are increased, and a space to install the temperature sensor needs to be reserved.
In regard to the space to install the temperature sensor, an image forming apparatus utilizes a measurement temperature of another member abutting the photosensitive element, instead of the temperature of the photosensitive element surface (for example, see Japanese Patent Application Laid-open No. 2003-043757), or an image forming apparatus uses a noncontact temperature sensor (thermopile).
However, any of these conventional image forming apparatuses needs the temperature sensor as ever, which causes the increase in the manufacturing cost of the image forming apparatus.
When the temperature sensor is used, the detected temperature can frequently have a value around 44° C. after a lapse of the driving time α (α is about 55 minutes) of the photosensitive element in the case of the room temperature of 28° C., for example, depending on detection accuracy of the temperature sensor, because the temperature of the photosensitive element surface near 44° C. is near a threshold for the image defect region, as shown in FIG. 10. Therefore, a printing operation is often suspended, which is inconvenient for the user.
There is a technique that enables to set a printable (start) temperature lower than a suspend temperature, thereby reducing the number of time of the suspend operations. For example, a case in which the printable temperature is set to a temperature that is 1° C. lower than the suspend temperature (44° C.) is examined. The driving of the photosensitive element needs to be stopped for about 10 minutes as indicated by γ in FIG. 11, to lower the temperature of the photosensitive element surface by 1° C. from 44° C. to 43° C. as indicated by β in FIG. 11; meanwhile, the printing operation is suspended, and it is inconvenient for the user.